what is the average pH of natural rain Solution Most rainwater has a pH of 5.6 to 5.8, simply due to the pressence of carbonic acid (H2CO3). The latter is formed from dissolved CO2 gas and H2O. The source of the CO2 is the atmosphere, which presently contains about 380 ppm CO2. Values of pH below 5.6 or so are considered \"acid rain\", values above are \"basic rain\". These names are somewhat in conflict with the common pH scale, for which 7 is \"neutral\", below 7 is \"acid\" and above 7 is \"base\". As far as rain in concerned, abnormal acidity or bacisity is relative to CO2-H2O equlibrium values of 5.6 to 5.8. There are both natural and non-natural sources of materials that cause rain pH to deviate from the CO2-H2O equlibrium values. .